4 Answers
4

I also agree with Nanne but will two suggest two other ways of monitoring your data connection...

You can add the Power Control widget to the home screen. It comes stock with Android and it will lets you easily toggle the Wifi among other things. When Wifi is on, 3G will be off and you'll have a visual representation right there. There are 3rd party version of this widget that actually let you toggle the 3G connection specifically.

If you have a limited data plan as you do, beside just being diligent about monitoring your data connection, you can automate the monitoring process with something like PhoneUsage Pro. This app allows you to set daily, weekly or monthly limits for Data usage and it will notify you when you hit those limits. Since theres sometimes a lag in registering data you could set up a notification to let you know when you've used 750 mb of 3G data in a month and then you'll know it's time to start using Wifi more frequently.

In addition to Nanne's answer, I believe Android also defaults to using WiFi when both connections are enabled. So if you have a WiFi network saved, your phone will use it when you return so you don't need to keep checking. (This assumes WiFi is enabled and there are no network problems, of course).

You can also shut off data use by going to Settings -> Wireless and network -> Mobile networks and un-checking Use packet data or similar.

Nanne is correct: your status bar should show you any network connections. If you really want to ensure that you're conserving your 3G bits, install APNdroid and keep data disabled except when you really need it. APNdroid gives you a nice screen widget.